Direct talk

Edited by Colleen Telling | Dec 01, 2005

In 1973, researcher Bob Metcalfe, Xerox Corp., developed a way to link computers to printers, thus creating a physical method of cabling, known today as Ethernet. The SERCOS interface — also known as Serial Real-Time Communications System — is an open, digital interface for communication between a digital controller and peripherals, such as I/Os, digital servo drives, and actuators. Together, they provide a powerful new motion bus for numerically controlled machines and systems.

Two German organizations, ZVEI and VDW, created SERCOS in the 1980s as a digital drive interface for advanced machine-tool applications to replace the +/-10 V analog interface. Since 1995, it has been the only internationally standardized (IEC 61491) motion control and I/O interface. Its applications have expanded to all types of automated machines, including machine tools, printing presses, packaging machines, and robots.

There are many other digital drive interfaces on the market. However, many are proprietary, and therefore, only work with the manufacturer's drives; others are “opened,” but are developed and controlled by a single manufacturer, with just a few firms selling products using that bus. In contrast, more than 50 control and 30 drive manufacturers worldwide offer products based on the SERCOS interface standard.

Improving the interface

Industrial Ethernet is the de facto standard for manufacturing information networking and is now reaching the servo drive level. However, synchronizing multiple drives requires a high degree of determinism, which is available in SERCOS, but not industrial Ethernet. While Ethernet is not deterministic, it does offer high bandwidth and low hardware costs. The best-of-both worlds solution is to combine SERCOS with industrial Ethernet as the physical layer. SERCOS III fulfills this need, running at up to 100 Mbit/sec. (By comparison SERCOS I operates at 2 and 4 Mbit/sec and SERCOS II at 2/4/8/16 Mbit/sec.)

SERCOS III maintains the hardware determinism of earlier SERCOS versions operating with submicrosecond jitter to provide precise real-time control. Additionally, an optional IP channel for TCP/IP (transmission control protocol/Internet protocol) is placed under control of the motion bus.

Further, SERCOS III maintains the protocol structure and the many profile definitions (parameters and functions) of the original SERCOS, which more than 300,000 applications successfully employ. This provides 500 standardized parameters, lets users link to existing communication infrastructures, makes new features possible, and decreases hardware costs.

One special feature in SERCOS III is direct communication between slaves, allowing them to exchange data in one communication cycle without involving the master. This is possible because of Ethernet physics' full, duplex characteristics.

Defining traits

In SERCOS, data is transmitted via a group of telegrams. The first step involves the controller (master) broadcasting a Master Synchronization Telegram (MST) at the start of a communications cycle to set up and synchronize the timing sequence. Then, the master transmits a Master Data Telegram (MDT) once each cycle, sending data such as command values to the slaves (drives, I/O). Next, the slaves return an Amplifier Telegram (AT) to the master, reporting on speed, torque, and position. Each slave repeats the AT and inserts its data in the appropriate slot.

While operating data is communicated in real time, a service channel is available that transmits nonreal-time data without disturbing the synchronous data transfer. Each telegram provides space for sending two bytes of service channel data (four bytes in SERCOS III). An additional, optional IP channel can be added to transfer nonreal-time data (such as TCP/IP and UDP/IP) via standard Ethernet frames. The IP channel's job is to directly transmit telegrams from superior networks (office, wide area) “up/down to” the drive or I/O. Transmission occurs in a separate channel and therefore, does not affect real-time data exchange between devices. End users can adapt cycle times and separate real time and nonreal-time channels for particular applications.

Tolerates faults with a double-ring signal flow structure in case of a break in the ring

Allows hot plugging for connecting and removing nodes during operation

Provides safety protocol for drive-integrated safety functions

Can operate at half the previous SERCOS minimum cycle time (from 62.5 to 31.25 µs)

Topology

Like the original SERCOS, SERCOS III has a ring structure. The difference is that SERCOS III employs a single daisy-chained Ethernet cable, which has a double-ring structure regarding signal flow for redundant data transfer. Therefore, if the ring breaks at any point, communication and manufacturing can continue, while an integrated diagnostics tool signals the break to an HMI.

Besides the double-ring, a line structure (single ring) is also possible in SERCOS III. While this doesn't offer redundancy, it does save a wire connection.

Centralized and decentralized drive concepts

SERCOS III halves the minimum cycle time of SERCOS from 62.5 to 31.25 µs, due to industrial Ethernet's high bandwidth. Up to 254 devices can be connected on a ring, and numerous rings can be implemented. As a result, SERCOS III supports both centralized signal processing and decentralized drive concepts. With centralized concepts, the drive closes only the current loop, whereas all other axes' loops are implemented in the central control electronics. In a decentralized configuration, the drive closes all control loops.

Synchronizing motion controls

SERCOS III specifies a profile for distributed control in modular machines and systems, allowing controller-to-controller synchronization between motion controls. This allows individual machine modules to be integrated into communication networks in real time without writing special software. The SERCOS C2C (Controller-to-Controller synchronization and communication) profile standardizes this type of communication and supplements existing SERCOS device profiles for servo drives and remote I/O devices. Typical profile applications are in printing, packaging, and processing machines, and machine tools that require special control systems and synchronization, such as gantry axes or rotary transfer tables.

Chips give options

Compared to SERCOS I and II, which utilize ASICs, SERCOS III employs standard modules such as FPGAs or General Purpose Communication Controllers (GPCC), thus reducing costs per node. To further decrease prices, industrial Ethernet connectors and copper-wire cables are used in place of fiber-optic cables and couplings; however, fiber optics will be available in SERCOS III for special applications.

FPGA controllers

A software core (SERCOS III IP) lets system manufacturers combine SERCOS III hardware and their logic components in one common FPGA, which includes all hardware functions such as timing, synchronization, and processing of cyclic and non-cyclic data built on two integrated Ethernet MACs. FPGAs from Xilinx and Altera are supported.

GPCC controllers

The SERCOS core has been integrated into a General Purpose Communication Controller (GPCC) — the Hilscher netX 500 — an ARM9260-based System-on-Chip network controller. NetX enables end users to implement control and drive devices that can be adjusted to one of several available industrial Ethernet protocols through appropriate driver software. This benefits OEMs as they will handle just one type of wiring and end users who won't need various hardware configurations for each industrial Ethernet protocol in a plant.